Study measures genetic contribution to Alzheimer's disease

Alzheimer's disease (AD) is 'highly heritable', and genetic factors may also influence the timing of the condition, say US researchers. The team, based at the University of Southern California, studied 392 pairs of twins in which one or both had AD. Their results, published in the Archives of General Psychiatry, suggest that genes could play an even more important role in the condition than previously thought.  

AD is caused by the gradual death of certain brain cells, especially in the areas involved in memory. Researchers have already identified mutations in three genes that raise the risk of the 'early onset', familial form of the disease, which usually first affects people in their fifties. They have also linked variants of two other genes - called ApoE4 and ubiquilin-1 - to an increased risk of the more common, late onset form of the disorder. The latest study sought to measure the genetic and environmental contribution to the latter type of AD, by looking at the Swedish Twin Registry.

In twin studies, scientists compare identical twins, who have identical genes and similar backgrounds, with non-identical twins - who also share similar backgrounds, but are no more genetically alike than ordinary siblings. In this way, they can estimate how much of the variation seen for a particular characteristic is accounted for by genes - a measure of 'heritability'. A trait that is completely controlled by genes, for example eye colour, has a heritability of 100 per cent. At the other extreme, a characteristic that has nothing to do with genes, such as a scar, will have a heritability of zero. Previous research indicated that late-onset AD has a heritability of at least 60 per cent.

The Californian team estimate that the heritability of AD is between 58 and 79 per cent. They also found evidence that genes influence the age at which the disease first appears: in pairs where both twins had AD, the age of onset differed by an average of 8.12 years for non-identical twins, but only 3.66 years for identical twins. However, the researchers stress that 'non-genetic risk factors also play an important role and might be the focus for interventions to reduce disease risk, or delay disease onset'.